Debian provides the
LTS
version of Tor. Note that this might not always give you the latest stable
Tor version, but you will receive important security fixes. To make sure
that you're running the latest stable version of Tor, see option two below.

Do not use the packages in Ubuntu's universe. In the past they have
not reliably been updated. That means you could be missing stability
and security fixes.

Raspbian is not Debian. Tor might run fine on the Raspberry Pi 2 / 3
but not the first generation Pi.
These packages might be confusingly broken for Raspbian users, since Raspbian
called their architecture armhf but Debian already has an armhf. See this
post for details.

Admin access:
To install Tor you need root privileges. Below all commands that need to be run
as root user like apt and dpkg are prepended with '&num;',
while commands to be run as user with '$' resembling the standard
prompt in a terminal. To open a root terminal you have several options:
sudo su, or sudo -i, or su -i.
Note that sudo asks for your user password, while su expects
the root password of your system.

GPG:
GNU Privacy Guard version 2.1 is needed for
this guide. If you are using an older version, consider upgrading to gnupg2
or replace 'gpg2' below with gpg --keyserver hkp://pool.sks-keyservers.net
since the keyserver option was mandatory for older versions.

apt-transport-tor:
To use source lines with https:// in /etc/apt/sources.list the
apt-transport-https package
is required. Install it with

&num; apt install apt-transport-https

to enable all package managers using the libapt-pkg library to access metadata
and packages available in sources accessible over https (Hypertext Transfer Protocol Secure).

sources.list:
You'll need to set up our package repository before you can fetch Tor. First,
you need to figure out the name of your distribution. A quick command to run is
lsb_release -c or cat /etc/debian_version.
If in doubt about your Debian version, check
the Debian website.
For Ubuntu, ask
Wikipedia.

I run
and want

You need to add the following entries to /etc/apt/sources.list
or a new file in /etc/apt/sources.list.d/:

deb https://deb.torproject.org/torproject.org jessie main

Then add the gpg key used to sign the packages by running the following
commands at your command prompt:

Now Tor is installed and running. Move on to
step two of the
"Tor on Linux/Unix" instructions.

The DNS name deb.torproject.org is actually a set of independent
servers in a DNS round robin configuration. If you for some reason cannot
access it you might try to use the name of one of its part instead. Try
deb-master.torproject.org,
mirror.netcologne.de or
tor.mirror.youam.de.

To use Apt with Tor the according apt transport needs to be installed:

&num; apt install apt-transport-tor

Then replace the address in the lines added before with, for example:

&num; For the stable version.
deb tor://sdscoq7snqtznauu.onion/torproject.org <DISTRIBUTION> main
&num; For the unstable version.
deb tor://sdscoq7snqtznauu.onion/torproject.org tor-nightly-master-<DISTRIBUTION> main

Now refresh your sources and try if it's still possible to install tor: